Since the 1939 work of Bennett, the father of soil conservation, the world has seen a host of erosion control manuals, most of them in English or Spanish and describing practical experience, technical principles, mechanical (and sometimes biological) methods to be used, and a series of practical recipes that have been adopted with varying degrees of success in specific regions. However, there have been few authors who, having seen at first hand the relative ineffectiveness of the generally recommended techniques, have been ready to re-examine the erosion control principles that Bennett developed for the very specific environmental, social and economic conditions of the large-scale, mechanized cropping of groundnut, cotton, tobacco and cereals, all providing little ground cover, that the European immigrants introduced into the semi-arid Great Plains of the United States of America during the Great Depression of the 1930s. Bennett's approach to soil conservation (based on draining runoff water from cultivated fields along gently sloping channels to designated outlets) was then applied, with no prior testing, in totally different circumstances (for example among small subsistence farmers in tropical upland areas)... with the very indifferent results that have been seen by all.
Science has made giant strides since Bennett's day.
Firstly, it has been discovered that the kinetic energy of raindrops can lead to degradation of cultivated soils. Risks of runoff and erosion can therefore be cut by introducing production systems that provide better ground cover (Ellison 1944, Stallings 1953, Wischmeier and Smith 1960 and 1978, Hudson 1973, Roose 1977a, etc.).
Secondly, people have realized that there are many different processes in soil degradation and erosion, with a variety of causes - and a similar variety of sometimes contradictory factors involved in any action to alter them. Treatment of sheet erosion has, for instance, sometimes increased the risk of landslides (as can happen with marls).
Thirdly, differences in physical landscapes and in the social and economic conditions of effective application of erosion control methods are better analysed today. The erosion crises facing large-scale, modern landowners in temperate zones are no longer treated in the same way as the subsistence problems of poor, densely-populated communities clinging to tropical hillsides.
Instead of simply describing schemes that have worked in one specific place, today one has to learn to assess different conditions and work with, rather than against, the forces of nature; for example, by progressively modifying the slope of a hillside by slowing down sheet runoff and using farming techniques that will gradually terrace the land, instead of tearing at mountains with powerful bulldozers to produce often unstable and expensive-to-maintain infrastructure.
The author would like to remind agricultural experts that erosion control is not the exclusive domain of specialists working to rehabilitate land degraded because it has been more mined than farmed, but must incorporate the viewpoints of the land-use planner responsible for water and soil fertility management in the development of cropping systems that are profitable, sustainable, and safe for rural and urban environments.
Since the 1980s there has been much criticism of the failure rate of most programmes incorporating erosion control.
It is now recognized that there are two spheres in erosion control:
• The State sphere, with the government responding to disasters and sending in engineers to stop landslides, control torrents, replant mountains with trees, or harness watercourses that threaten structural works, lines of communication, inhabited areas, irrigation schemes and dams through siltation. In the public interest, representatives of the central authorities insist on water control in the rural environment. It is expensive and upsets the farmers, but is the only way of controlling the quality of water supplies (the off-site perspective), and only the State is in a position to engage in such large-scale mechanical undertakings.
• The farming sphere of land protection (the on-site perspective), which can be assured only by the rural community, so long as it is helped in making a correct diagnosis of the causes of the erosion crisis and the best ways of improving environmental protection, biomass production and living standards.
It is essentially on this latter sphere - that of water, soil fertility and biomass management (GCES), or land husbandry - that this work would like to focus, taking stock especially of research by French-speaking soil, agricultural and geographical experts (particularly from ORSTOM and CIRAD), who have worked mainly in Africa, where problems develop much faster than in Europe. After all, the work of English-speaking experts in this sphere is already well known (Wischmeier and Smith 1978, Hudson 1992).
The author presents a personal and intentionally confrontational point of view, offering a new and more constructive approach to the problems small farmers face in their battle with the degradation of their land. This is not a manual with clear-cut remedies for each and every erosion problem, but a work that should allow research experts, teachers and agronomists in the field to appreciate differences in situations, diagnose the causes of crises, and propose a range of technical solutions from which a small rural community (a family, a ward, a village, a slope, a hillside or a micro-watershed) can choose the technological package best suited to its particular needs. Rather more "instruction-oriented" material for training extension agents (Dupriez and De Leener 1990, Inades 1989) and more technical manuals on torrent control and landslides (Heusch 1988, CEMAGREF documentation) or improving soil fertility (Pieri 1989) are available elsewhere.
This document has been used for eight years as a basis for courses on "Land Husbandry as an Instrument in Land Management" given to 700 agricultural or forestry engineers at CNEARC and ENGREF in Montpellier, in France, and ANDAH in Haiti, as well as 50 senior water technicians at ETSHER in Ouagadougou, in Burkina Faso. It is hoped that future editions will be enriched with readers' comments and details of new experiences. It will have met its aim if it provides large numbers of land-use planners and agronomists with pointers for developing intensive and sustainable farming systems suited to specific environmental situations and social and economic contexts.